4D extrema detection involves identifying either peaks or valleys of the DoG saliency operator \(local~\underset{x,y,z,\sigma }{\arg \max }\lbrace  \Vert  DoG(x,y,z,\sigma ) \Vert  \rbrace \) , approximating maxima of the LoG operator \(local~\underset{x,y,z,\sigma }{\arg \max }\lbrace  \Vert  \nabla ^2I(x,y,z,\sigma ) \Vert  \rbrace \)  which represent informative keypoint coordinates in scale-space. This is performed using three DoG volumes computed at consecutive scale \(\sigma \)  increments, identifying voxel locations that are either higher or lower than all \(27+26+27=80\)  adjacent voxels. The method used here is inspired by the environment used in computing the 2D LBP descriptor [1]}, [2]}.
